1. Field of the Invention
The present invention relates to polyesters which are suitable for various applications including the manufacture of formed articles, such as containers. More specifically, the present invention relates to a polyester comprising the residue of a dicarboxylic acid component comprising isophthalic acid and terephthalic acid, a glycol component comprising ethylene glycol, and an aromatic diol derivative.
2. General Background and Description of Related Art
It is well known in the art that poly(ethylene terephthalate) (PET) is useful for many packaging applications. It is also well known and practiced that PET or modified PET is the polymer of choice for making beverage and food containers, particularly carbonated beverage containers. Furthermore, it is known that PET can be modified with additional diacid and/or glycol comonomers on a commercial scale, e.g., isophthalic acid (or dimethyl isophthalate), 1,4-cyclohexanedimethanol (CHDM), and the like.
Modifying PET with additional comonomers may improve some of the physical properties of the resulting polyesters and provide particularly desired properties in an article formed from the polyester such as increased barrier and improved crystallization and processability. Moreover, comonomers may provide these beneficial properties to the resulting PET when used alone or in combination with other monomers. CHDM, for example, improves the ease of processing of a PET resin, but may unfortunately decrease the barrier properties of a resulting container made with the PET resin. Isophthalic acid (IPA), for example, increases the barrier of PET, but may unfortunately decrease the processing window in which acceptable containers can be formed. Therefore, by including CHDM and IPA comonomers in a PET resin, for example, an article may be formed from the PET resin which has improved barrier and easier processing properties.
Using a particular aromatic diol derivative as a copolymer to improve barrier properties has also been disclosed. For example, U.S. Pat. No. 4,440,922 describes polyesters made from phenylenedi(oxyacetic acid), which is also known as and includes resorcinol oxydiacetic acid, hydroquinone oxydiacetic acid, and catechol oxydiacetic acid. However, this reference does not disclose a polyester having the residues of terephthalic acid, isophthalic acid, ethylene glycol and an aromatic diol derivative in mole % amounts to improve UV absorption.
U.S. Pat. Nos. 4,552,948, 4,663,426, and 5,030,705 describe the use of copolyesters containing phenylenedi(oxyacetic acid) for containers. These copolyesters have low permeability. However, because of the high level of modification, these copolyesters are difficult to crystallize. The poor crystallization behavior of these copolyesters makes them difficult to dry and limits the amount of strain induced crystallization that occurs during container fabrication. Low levels of crystallinity in the containers often result in poorer mechanical properties and lower gas barrier. Moreover, these copolyesters may not exhibit improved UV absorption and do not include residues of terephthalic acid, isophthalic acid, ethylene glycol and an aromatic diol derivative in the mole % amounts of this invention.
U.S. Pat. No. 5,239,045 describes copolyesters containing terephthalic acid, ethylene glycol and 0.5 to 4.5 mole % of phenylenedi(oxyacetic acid). However, a polyester having the residues of terephthalic acid, isophthalic acid, ethylene glycol and an aromatic diol derivative in mole % amounts to improve UV absorption is not disclosed. U.S. Pat. No. 4,959,421 describes blends of PET with copolyesters containing isophthalic acid, naphthalenedicarboxylic acid and phenylenedi(oxyacetic acid), but not terephthalic acid, isophthalic acid, ethylene glycol and an aromatic diol derivative in mole % amounts according to the present invention.
Heretofore, polyesters (particularly PET and modified PET) have not displayed significantly improved UV absorption and current polymers are insufficient for all UV absorbance needs. UV absorption is a very beneficial and desirable physical property for polyesters. UV absorption provides articles molded from a polyester (e.g., film and containers) with improved capability to protect the contents of the article, such as food and beverages, from UV degradation. A comonomer that provides improved UV absorption to a resulting polyester has not been used in conjunction with other comonomers to provide a polyester and an article made from such polyester with sufficient barrier, UV absorption and processing properties. All UV absorption efforts have been by adding non-reacting blends of commercial UV absorbers. The commercial UV absorbers volatilize during extrusion which results in wasted material, operator discomfort, and plating out of residues.
Accordingly, there remains a need in the art for an easily processable polyester which displays improved barrier, acceptable crystallization even without crystallization additives, and improved UV absorption. Moreover, there is a need for polyester articles having improved UV absorption to protect against UV degradation.